Thrombin generation in patients with acquired haemophilia and clinical bleeding risk

Sensitive Measurement of Clinically Relevant Factor VIII Levels in Thrombin Generation Assays Requires Presence of Factor XIa

BACKGROUND

 Hemophilia A (HA) is characterized by decreased or absent factor VIII (FVIII) activity. Current FVIII assays are based on clotting time and thus only provide information about the initiation of coagulation. In contrast, thrombin generation assays (TGAs) can be used to measure the full coagulation spectrum of initiation, propagation, and termination that provide information on the whole course of thrombin generation and inhibition. However, the commercially available TG kits lack sensitivity for measurements of hemophilia plasma within lower FVIII ranges, which is essential for explaining differences in bleeding phenotypes in hemophiliacs at clinically low levels of FVIII.

AIMS

 Optimization of the TGA for measurements of low FVIII levels in severe HA patients.

METHODS

 TGA measurements were performed in severe HA pooled plasma (n = 10). Investigations of several preanalytical and analytical variables of the assay were performed in a stepwise process and adjusted based on sensitivity toward intrinsic coagulation activation.

RESULTS

 TGA initiated by tissue factor (TF) alone at varying concentrations was unable to significantly differentiate between FVIII levels below 20%. In contrast, TGA activation with low concentrations of TF in presence of FXIa appeared to be highly sensitive for FVIII changes both in high and low ranges. In addition, a representative TGA curve at trough levels could only be produced using the dual TF/FXIa TGA.

CONCLUSION

 We propose a critical optimization for the setup of the TGA for measurements in severe HA plasma. The dual TF/FXIa TGA shows increased sensitivity, especially in lower FVIII ranges, which allows for better individual characterization at baseline, prediction of interventions, and follow-up.

In vitro assessment, using thrombin generation, of the applicability of prothrombin complex concentrate as an antidote for Rivaroxaban

BACKGROUND

Rivaroxaban has been approved as an antithrombotic agent for prevention of venous thromboembolism with specific indications. At present no antidote is appointed and no guidelines have been formulated for the measurement of Rivaroxaban reversal.

OBJECTIVES

In the present study, we have evaluated the influence of prothrombin complex concentrate (PCC) on the anticoagulant effects of Rivaroxaban as measured by prothrombin time (PT) and thrombin generation tests (TGTs).

METHODS

Plasma and whole blood samples from healthy volunteers were spiked with Rivaroxaban (up to 800 μg L(-1) ) and PCC was added to these samples in concentration ranges as used clinically to reverse the effects of vitamin K antagonists. PT, endogenous thrombin potential (ETP) and calibrated automated thrombography (CAT) assays were performed with varying tissue factor (TF) concentrations.

RESULTS

Addition of PCC to Rivaroxaban-spiked samples did not result in normalization of PT and TGT lag time/T-Lag in ETP and CAT, respectively. In contrast, normalization of ETP and CAT area under the curve did occur. However, the response to PCC addition was strongly TF concentration dependent and in whole blood less PCC was required for Rivaroxaban reversal as compared with plasma.

CONCLUSIONS

Prothrombin complex concentrate does not neutralize the lengthening effect on PT and TGT lag time/T-Lag of Rivaroxaban anticoagulated blood in vitro; however, total thrombin potential could be normalized. Response of the different TGTs in this respect is assay condition dependent. Therefore, prospective studies are needed to clarify which assay condition and parameter describes in vivo hemostasis best in patients on Rivaroxaban who are treated with PCC.

Potentiation of thrombin generation in hemophilia A plasma by coagulation factor VIII and characterization of antibody-specific inhibition

Development of inhibitory antibodies to coagulation factor VIII (fVIII) is the primary obstacle to the treatment of hemophilia A in the developed world. This adverse reaction occurs in 20-30% of persons with severe hemophilia A treated with fVIII-replacement products and is characterized by the development of a humoral and neutralizing immune response to fVIII. Patients with inhibitory anti-fVIII antibodies are treated with bypassing agents including recombinant factor VIIa (rfVIIa). However, some patients display poor hemostatic response to bypass therapy and improved treatment options are needed. Recently, we demonstrated that fVIII inhibitors display widely variable kinetics of inhibition that correlate with their respective target epitopes. Thus, it was hypothesized that for antibodies that display slow rates of inhibition, supplementation of rfVIIa with fVIII would result in improved thrombin generation and be predictive of clinical responses to this novel treatment regimen. In order to test this hypothesis, 10 murine monoclonal antibodies (MAbs) with non-overlapping epitopes spanning fVIII, differential inhibition titers, and inhibition kinetics were studied using a thrombin generation assay. Of the 3 MAbs with high inhibitory titers, only the one with fast and complete (classically defined as "type I") kinetics displayed significant inhibition of thrombin generation with no improvement upon supplementation of rfVIIa with fVIII. The other two MAbs that displayed incomplete (classically defined as "type II") inhibition did not suppress the potentiation of thrombin generation by fVIII. All antibodies that did not completely inhibit fVIII activity demonstrated potentiation of thrombin generation by the addition of fVIII as compared to rfVIIa alone. In conclusion, fVIII alone or in combination with rfVIIa corrects the thrombin generation defect produced by the majority of anti-fVIII MAbs better than single agent rfVIIa. Therefore, combined fVIII/rfVIIa therapy may provide better hemostatic control than current therapy in some patients with anti-fVIII inhibitors.